Timing
Seeds are usually collected only once they are ripe. Signs of ripeness include splitting and opening of the seedpods or husks, and a darkening in colour of the seeds themselves. Seeds within fleshy fruits usually ripen at the same time as the fruits.
There are some exceptions to the rule of collecting seeds only when ripe.
Seeds of trees and shrubs that produce catkin-type flowers (left) are best collected while still unripe and green. The catkins can be hung upside down in a paper bag to collect the seeds as they ripen. If left on the plant, they would disperse in the wind so rapidly, that you would have no chance of collecting them.
Seeds of some other trees develop germination inhibitors as they ripen. These are often trees with fleshy fruits or berries such as Sorbus (rowan), Tilia (lime) and Crataegus (hawthorn). These fruits are best gathered early, before they are fully coloured, and certainly before any hint of shrivelling. The seeds inside the fruits should still be green. This will ensure better germination the following spring.
Choose a dry day for seed collection, so that the seeds do not become damp.
Choice of plants
Only take seed from healthy looking plants. Sickly or non-vigorous plants could have diseases such as viruses. These can sometimes be transmitted via seed. This means that the resulting seedlings would be similarly non-vigorous.
Obviously, it is best to choose plants that you wish to increase your numbers of, and which have seeds that are appropriately ripe.
Bear in mind that many hybridised plants will not come true to type from seed. Unlike with cuttings, where you get an exact clone of the parent plant, seedlings will be genetically different from their parents, and may have altered colours or growth habit.
Some seedpods, for example on paeonies (left), can contain both fertile and sterile seeds. The sterile seeds are often pinkish or red in colour, whereas the fertile seeds often start off whitish, and then ripen to black.
Removal of seeds from the parent plant
Cut off ripe looking seedpods, seed cases, fruits or cones from the parent plant.
Large trees can be shaken to dislodge fruits and seeds. A blanket can be laid under the tree to catch the seeds as they fall.
Wind-fallen fruits and cones can be picked up from the ground, but do take care that they are not infested with maggots or rots
Seed cleaning
Dry seeds often need nothing more than the picking off and teasing apart of the cotton wool-like substance that holds them together. Populus (poplar) and Salix (willow), both catkin-producing trees, are examples. Clematis seed heads (left) have a similar cotton wool-like appearance.
Seedpods can be spread out on a tray to dry. When they have completely split and shed their contents, they can be rubbed with the hands, to separate out and crush the chaff. The dry mixture can then be sieved and picked through, to isolate the seeds.
Fleshy pods containing large seeds, such as broad beans (left), can be hung upside down, still on the plant, to dry. When the pods and leaves are shrivelled and completely dry, the beans can be removed by hand and left to dry further, if necessary.
Winged seeds, such as Fraxinus (ash) or Acer (maples and sycamores), can either be stored with the wing cases still on, or with wings rubbed off, so that they take up less space and are easier to plant.
Nuts should have the outer husks removed, but the shell left on.
Fleshy fruits, such as peaches, apples, or Sorbus berries (mountain ash; rowan), should have the fleshy portion removed to expose the inner seeds. Apple pips and peach stones can be extracted with a knife and then rinsed off. Berries can be mashed in a sieve and held under running water so that the pulp washes away. The seeds should then be left to dry on a paper towel in a warm room.
Cones can be dried in a warm room. They will gradually open, until the seeds are released.
Seed storage
Remove any shrivelled or damaged looking seeds before storing.
Many seeds are best stored dry, at fridge temperatures. This provides optimal conditions for avoiding rots, and for maximising the viability of the seeds by slowing down their metabolism and reducing their rate of deterioration. The best way to keep seeds dry in the fridge is to store them in paper bags or envelopes, which are then placed inside a plastic bag to protect them from the dampness often found in fridges.
Large, oily seeds, such as walnuts (Juglans) and Quercus (oak) acorns, cannot be completely dried out before storage. Fleshy Magnolia seeds should not be dried either, as they lose the ability to take up water once dried, and then fail to germinate. Such seeds are best stored in plastic bags of moist sand, peat substitute, or fine vermiculite (left).
Dormancy
Many hardy tree seeds have a degree of inherent dormancy, which can be overcome by appropriate storage techniques. This makes germination much easier the following spring. Such seeds require the winter weather that they would normally experience when shed from the plant in the autumn, to break their dormancy. The frost and the cold break down the hard seed coast of trees such as Acacia and Robinia.
Cold also has an effect on the internal hormonal balance of some seeds, helping to overcome the inherent physiological dormancy that is apparent in some species. Acer and Arbutus (strawberry tree) are two examples. This kind of dormancy can be overcome by combining storage with the technique of cold stratification.
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If seeds are to be stored long term, then the addition of a dessicant such as silica gel or calcium chloride (sold in many DIY stores) will help to keep them dry for longer. Freezing seeds will also preserve them for longer - often for several years. But germination is usually best when the seeds are sown fresh, declining with increasing length of storage.
Although various means can be used to overcome the deepening dormancy that can occurs in storage (see RHS Propagating Plants by Alan Toogood, ISBN 0-7513-0365-8 for more details), the loss of water from the seeds during lengthy storage periods will reduce their viability over time.
Maya Albert